In various industries such as the power generation industry, there is a need to inject chemical solutions into flowing process streams at elevated pressures and temperatures for various purposes. In particular, it is necessary to inject solutions of noble metal containing chemicals, such as Na2Pt(OH)6, into the feedwater piping of boiling water nuclear reactors to aid in inhibiting intergranular stress corrosion cracking of susceptible structural materials in the reactor vessel in the presence of hydrogen.
As reported by Hettiarachchi and Diaz, the noble metal chemical solution Na2Pt(OH)6 is added to the feedwater piping of boiling water nuclear reactors over a 10 day period. Such 10 day injection periods are repeated during each subsequent yearly fuel cycle. For boiling water nuclear reactors with longer fuel cycles, the 10 day applications are conducted on an annual basis. The total mass of noble metal injected annually is also limited to a fixed value by an industry consensus standard described by Garcia et al. Because of a phenomenon known as “crack flanking”, described by Andresen and Kim, it is advantageous to inject the noble metal chemical over the entire operating period of a fuel cycle, not just during an annual 10 day period. Active metering pumps used for these 10 day injections, such as positive displacement pumps, have experienced maintenance problems due to interaction with the noble metal chemicals such as Na2Pt(OH)6 and are not optimum for long term injection.
A boiling water nuclear reactor that follows the industry consensus recommendation will typically add between 200 and 1, 200 gm of Pt (as Na2Pt(OH)6) each calendar year, depending on plant specific features such as fuel surface area and power rating. If the addition is made continuously at a constant rate over 365 days, the addition rate will vary between 3.8×10−4 and 2.3×10−3 gm (Pt)/min. If the feedstock is a 1% solution of Na2Pt(OH)6, the addition rate will be between 0.038 and 0.23 ml/minute (cc/m). The resulting concentration of Pt in the feedwater would be on the order of 10 parts per trillion. Accordingly, there has been a need in the nuclear industry for a chemical injection system that does not employ active pumps and is capable of adding small, metered amounts of noble metal chemicals, such as Na2Pt(OH)6, into the feedwater during the entire fuel cycle.
U.S. Pat. No. 8,054,933 (Tran et al) describes a method of injecting chemicals into flowing nuclear reactor water streams teaching the use of positive displacement pumps, a process computer, various valves, chemical storage tanks, weighing scales and a source of deionized water. While this system is useful in injecting chemicals over short periods of time, it is quite complicated and not necessarily suited for trouble free injection of dilute solutions over longer periods of time.
U.S. Pat. No. 2,266,981 (Miller) discloses a method and apparatus for injecting chemicals into a natural gas pipeline operating at elevated pressures that does not use a pump. The apparatus teaches a fluid supply for storing the chemical to be injected, a pressure feed tank for pressurizing and injecting the chemical into the pipeline and a series of lines, manual valves and gauges for controlling the flow of chemicals from the supply tank into the feed tank and ultimately into the pipeline using gravity. The natural gas line pressurizes the pressure feed tank to the same pressure as the gas in the pipeline and gravity allows the solution in the pressurized tank to flow into the gas pipeline. This arrangement would not work in adding low flow rates of chemicals into a flowing water filed pipe; as the pressurizing gas above the liquid in the feed tank would eventually become saturated. Degassing of the feed solution within the flow restrictor (valve, capillary) would occur and alter the rate of injection precision. An active flow rate control is required to maintain a constant injection rate as the change in height of the feed solution drains the tank. U.S. Pat. No. 6,779,548 (McKeary) teaches a similar method as U.S. Pat. No. 2,266,981 (Miller) but adds a system for automatically controlling the quantity of chemical injected into a pressurized gas system by employing two tanks, one pressurized and one not pressurized. While this system could work well adding liquid to a gas process stream, it will not control a liquid addition to a liquid stream to the accuracy and precision required for very low flow rates required in Pt injection nuclear applications. Similar problems will occur as with U.S. Pat. No. 2,266,981 (Miller)
All injection patents researched for this application have some sort of active displacement component, do not account for dilution of the primary injection solution, have cover gas pressurization (that saturate the chemical injection solution), have active flow controls, or cannot yield very low flow rates (sub ccm) continuously over very long periods (months-year) without intervention.